Seismic prospecting for petroleum involves the creation of acoustic disturbances above, upon, or just below the surface of the earth, using explosives, air guns, or large mechanical vibrators. Resulting acoustic waves propagate downwardly in the earth, and partially reflect back toward the surface when acoustic impedance changes within the earth are encountered. A change from one rock type to another, for example, may be accompanied by an acoustic impedance change, so that the reflectivity of a particular layer depends on the velocity and density content between that layer and the layer which overlies it.
In early years, signal traces of the reflected acoustic waves were recorded immediately in the field as visible, side-by-side, dark, wiggly lines on white paper ("seismograms"). At present, the initial reproductions--in a digital format--are on magnetic tape, and finally are reduced to visible side-by-side traces on paper or film in large central computing facilities.
At such centers, sophisticated processing makes possible the distinction of signals from noise in cases that would have seemed hopeless in the early days of seismic prospecting. Until 1965, almost all seismic surveys conducted used an automatic gain control which continuously adjusted the gain of amplifiers in the field to account for decreasing amounts of energy from late reflection arrivals. As a result, reflection coefficients could not be accurately determined. However, with the advent of the expander circuit and binary gain amplifiers, gain of the amplifiers can now be controlled and amplitudes recorded precisely; this makes it possible to conserve not only the special characteristics of the reflections, but also their absolute amplitudes.
More powerful computers with array processors and economical floating point capabilities also now enable modern geophysicists to maintain control of the amplitude of all recorded signals. The "floating point" capability is especially effective in expanding computer work size by a large factor and in eliminating the need for computer automatic gain control. As a result of the above advances, reflections from many thousands of feet below the earth's surface can now be confidently detected and followed through sometimes hundreds of side-by-side traces, the shortening or lengthening of their corresponding times of arrival being indicative of the shallowing or deepening of actual sedimentary strata of interest.
Apropos of the above has been use of ultra-high amplitude anomalies in seismic traces to infer the presence of natural gas in situ. Seismic interpreters have used so-called "bright-spot" analysis to indicate several large gas reservoirs in the world, especially in the Gulf Coast of the United States. Such analysis is now rather common in the oil industry, but it is not without its critics. Not only cannot the persistence of such increased amplitude anomalies be taken as confirmation of the lateral extent of the gas reservoir, but also the anomaly itself (in some cases) may not represent reflections of a discontinuity of a gas-bearing medium and its over- or underlying associated rock strata.
However, the problem as to the degree an interpreter can rely on high-intensity anomalies, in these regards has recently been brought to manageable proportions. In the above-identified copending applications, it is taught that gas-bearing potential and the lithology of one host and cap rock strata can be accurately determined by: (1) obtaining field data in which the data of common centerpoints are associated with more than one source-detector pair, (2) indexing the data whereby all recorded traces are indicated as being a product of respective source-detector pairs of known horizontal offset and centerpoint location, (3) thereafter, associating high-intensity amplitude anomalies in the traces in a manner that allows determination of both gas-bearing potential and the lithology of the host and cap strata to a surprisingly accurate degree.
The present invention further improves the ability of the seismologist to correctly identify valuable characteristics, such as lithology and authenticate presence of hydrocarbonaceous fluids using certain statistically-aided occurrences in amplitude with offset of such high-intensity anomalies of the seismic records to differentiate the former from similarly patterned reflections of other types of configurations containing no gas accumulations. If source and receiver characteristics are constant and if the geological section does not change as the measuring points move over the surface of the earth, it is axiomatic that the reflection traces would also be constant. That is to say, there would be at the maximum, a constant scale factor difference between corresponding values on successive seismic traces. But experience shows that each trace is different from the preceding or the following trace. While the degree of coherence or correlation can be defined mathematically say, as being related to the cross-correlation coefficients generated between equivalent time windows of the compared traces, the present invention relates primarily to the use of a different type of measurement of trace similarity, viz., one that instantaneously qualifies whole portions of the gather (rather than on a trace-by-trace basis).